Validation of ACE-FTS satellite data in the upper troposphere/lower stratosphere (UTLS) using non-coincident measurements

[摘要] CO, O₃, and H₂O data in the upper troposphere/lower stratosphere (UTLS)measured by the Atmospheric Chemistry Experiment Fourier TransformSpectrometer (ACE-FTS) on Canada's SCISAT-1 satellite are validated usingaircraft and ozonesonde measurements. In the UTLS, validation of chemicaltrace gas measurements is a challenging task due to small-scale variabilityin the tracer fields, strong gradients of the tracers across the tropopause,and scarcity of measurements suitable for validation purposes. Validationbased on coincidences therefore suffers from geophysical noise. Twoalternative methods for the validation of satellite data are introduced,which avoid the usual need for coincident measurements: tracer-tracercorrelations, and vertical tracer profiles relative to tropopause height.Both are increasingly being used for model validation as they stronglysuppress geophysical variability and thereby provide an "instantaneousclimatology". This allows comparison of measurements between non-coincidentdata sets which yields information about the precision and a statisticallymeaningful error-assessment of the ACE-FTS satellite data in the UTLS. Bydefining a trade-off factor, we show that the measurement errors can bereduced by including more measurements obtained over a wider longitude rangeinto the comparison, despite the increased geophysical variability. Applyingthe methods then yields the following upper bounds to the relativedifferences in the mean found between the ACE-FTS and SPURT aircraft measurements in theupper troposphere (UT) and lower stratosphere (LS), respectively: for CO±9% and ±12%, for H₂O ±30% and ±18%, and for O₃±25% and ±19%. The relative differences for O₃ can be narroweddown by using a larger dataset obtained from ozonesondes, yielding a highbias in the ACE-FTS measurements of 18% in the UT and relative differencesof ±8% for measurements in the LS. When taking into account the smearingeffect of the vertically limited spacing between measurements of the ACE-FTSinstrument, the relative differences decrease by 5–15% around thetropopause, suggesting a vertical resolution of the ACE-FTS in the UTLS ofaround 1 km. The ACE-FTS hence offers unprecedented precision and verticalresolution for a satellite instrument, which will allow a new globalperspective on UTLS tracer distributions.

[学科分类] 大气科学